The present invention relates to an air-fuel ratio feedback control method of an internal combustion engine, and more specifically to an air-fuel ratio feedback control method using an electrical digital computer.
An internal combustion engine, in general, emits gases containing pollutants such as carbon monoxide (CO), nitrogen (NOx), unburned or partly burned hydrocarbons (HC). When these pollutants are to be cleaned using a three-way catalytic converter, it is required to very precisely control the air-fuel ratio within a range around the stoichiometric air-fuel ratio such that all of the three components, i.e., CO, NOx and HC can be removed effectively.
Therefore, an internal combustion engine employing the above-mentioned three-way catalytic converter usually adopts a method of controlling the feedback of air-fuel ratio responsive to signals from a concentration sensor (exhaust gas sensor) which detects the concentrations of particular components in the exhaust gas. Among many concentration sensors, an oxygen concentration sensor (hereinafter referred to as O.sub.2 sensor) for detecting the oxygen concentration has been extensively used for automobiles, such as a stabilized zirconia element or a titania element. When the air-fuel ratio in the atmosphere hovers around 14.5 (stoichiometric air-fuel ratio), the O.sub.2 sensor of this type exhibits suddenly changed electric properties. In other words, the O.sub.2 sensor detects the changes in the air-fuel ratio causing the electric signals thereof to change.
The O.sub.2 sensors, however, have undesirable characteristics in that the output voltage thereof greatly varies in response to a change in the temperature. The internal resistance of the O.sub.2 sensors and the electromotive force of the zirconia type O.sub.2 sensors exhibit great variation in temperature characteristics. Particularly, at low temperatures, since the internal resistance greatly increases, the O.sub.2 sensors become inactive and in this inactive condition the feedback control operation initiated in response to the output voltage of the O.sub.2 sensors cannot be executed. Therefore, in conventional air-fuel ratio control methods, the coolant temperature is always monitored, and if the coolant temperature becomes lower than a predetermined temperature, the feedback control (closed-loop control) of the air-fuel ratio is forcibly inhibited from executing and an open-loop control of the air-fuel ratio is begun.
In the above-mentioned conventional method, the coolant temperature is used for recognizing whether the O.sub.2 sensor is active or inactive. According to such a conventional method, when the coolant temperature sensor malfunctions, even when the O.sub.2 sensor is active, no air-fuel ratio feedback control is executed, or even when the O.sub.2 sensor is inactive, the air-fuel ratio feedback control is carried out in response to a false signal from the O.sub.2 sensor. Furthermore, according to the conventional method, since precise judgement whether the O.sub.2 sensor is active or not cannot be expected, the operative temperature range wherein the O.sub.2 sensor is active is restricted to a very small range. This is a serious problem for present day internal combustion engines whose exhaust gas temperature is controlled to be low, because in such engines, the O.sub.2 sensor must be operated under a low temperature condition in which it is considered to be inactive.